1. Field of the Invention
The invention relates to laser systems which utilize a Raman cell in order to (1) shift laser frequency and (2) shorten laser pulse duration.
2. Discussion
Two references which describe experiments by two teams of researchers using intracavity Raman oscillators will be discussed. As to the first reference, F. deRougmont, Ding Kong Xian, R. Frey, and F. Pradere, in "High Efficiency Pulse Compression With Externally Pumped Intracavity Raman Oscillators", Optics Letters, Volume 9, No. 10, Page 460, 1984, discuss a system shown in FIG. 1. In that figure, three frequencies of light are illustrated by three different types of drawing lines. The laser pumping beam, having a frequency of 14,404 CM.sup.-1, is illustrated by solid line 3. The first Stokes beam, 10,249 CM.sup.-1, is illustrated by dotted line 6. The second Stokes beam, 6,094 CM.sup.-1, is illustrated by dashed line 9. Mirrors 12 and 15 define the ends of a resonant cavity which is resonant at the first Stokes frequency. A Raman medium 18, in the form of gaseous hydrogen at 40 atmospheres pressure, is located within the resonant cavity.
Mirrors 21 and 24, contained within the resonant cavity, are reflective at the first Stokes frequency, but are transmissive at both the pumping frequency and the second Stokes frequency, as indicated by the passage of both solid line 3 and dashed line 9, through the mirrors 21 and 24. Other mirrors and detectors which were used to measure the three light beams are not shown in FIG. 1.
Two different types of excitation of the Raman medium 18 in FIG. 1 occur. In one type, a ruby laser 27 generates a pumping pulse which is injected into the resonant cavity as indicated by arrow 30. In the second type of excitation, a second, injection pulse, (of three millijoule energy, three nanosecond duration, and at the first Stokes frequency) indicated by arrow 33, is also injected into the cavity, just prior to the injection of the pumping pulse 30. The injected pulse 33 is derived from the pumping pulse by apparatus indicated by block 36, which splits off the injection pulse from the pumping pulse, shapes it and amplifies it.
The experimental results obtained by these four researchers are illustrated in FIG. 2. FIGS. 2A-D, comprising the left column of the figure, illustrate intensity-versus-time plots of the three laser beams in FIG. 1, where both the pumping pulse 30 and injection illustrated by arrow 33 in FIG. 1 occur. The right hand column, including FIGS. 2A'-D', illustrate comparable plots, but with pumping only (i.e., without injection.) Specifically, FIGS. 2A and A' illustrate the intensity-versus-time behavior of the pumping pulse 30, measured approximately at point 39 in FIG. 1. Further, the measurement was taken in both cases with hydrogen absent from the Raman cell 18. The reader will note that plots of FIGS. 2A and A' are virtually identical, consistent with the absence of Raman scattering because of the absence of hydrogen.
FIGS. 2B and B' illustrate the intensity of the pumping pulse 30, again at point 39, but in the presence of 40 atmosphere hydrogen in the Raman cell. As FIG. 2B indicates, the presence of the hydrogen causes a depletion in the pumping pulse (e.g., at region 42) and, further, a greater depletion is obtained in the presence of the injected pulse 33 in FIG. 1, as illustrated by FIG. 2B.
FIGS. 2C and C' illustrate the intensity of the first Stokes frequency, measured at approximately region 45 in FIG. 1. FIGS. 2D and D' illustrate the intensity of the second Stokes frequency, measured approximately at region 48 in FIG. 1. In these latter two figures, the duration of the second Stokes pulse is approximately two nanoseconds.
Therefore, this reference is viewed as illustrating the use of hydrogen as a Raman medium, the medium being contained within a cavity which is resonant at the first Stokes frequency for hydrogen, and from which the following shifting of wave length and pulse compression are obtained: a pumping pulse having a full width at half maximum of twenty five nanoseconds, and a frequency of 14,404 CM.sup.-1, was shifted and compressed into a second Stokes pulse of duration of approximately two nanoseconds, at a frequency of 6,094 CM.sup.-1. The former is shown in FIGS. 2A and A', while the latter is shown in FIGS. 2D and D'.
A second reference is R. Frey, A. deMartino, and F. Pradere, "High Efficiency Pulse Compression With Intracavity Raman Oscillators", Optics Letters, Volume 8, No. 8, Page 437, 1983. In that reference, the apparatus of FIG. 3 is discussed. Mirrors 50 and 53 define the ends of a cavity which is resonant at the pumping frequency, which is the same frequency as that in the previous reference, namely 14,404 CM.sup.-1. A ruby laser 56 injects optical energy into this resonant cavity. A Q switch 59 spoils the Q of the resonant cavity. Contained within this resonant cavity is a pair of Raman cells 62 and 64 containing pressurized hydrogen. Mirrors 67 and 70 are reflective at the pumping frequency, as indicated by the reflection of laser beam 73, but are transmissive to the first Stokes frequency illustrated by dotted line 76.
A second cavity, called a Stokes cavity, was created by mirrors 79 and 82. Mirror 82 is 100% reflective at the first Stokes frequency (which is 10,249 CM.sup.-1, as in the first reference) while mirror 79 has a four percent reflectivity at the first Stokes frequency, thus being very transmissive at this frequency.
FIG. 4 illustrates three pulse profiles obtained at the pumping and Stokes frequencies. FIG. 4A illustrates an intensity-versus-time profile of the pumping pulse with mirrors 79 and 82 removed. The intensity was measured approximately at point 85 in FIG. 3. FIG. 4B illustrates the pumping pulse intensity, but with mirrors 79 and 82 installed, thus providing a Stokes cavity between them. FIG. 4C illustrates the output at the first Stokes frequency, measured approximately at region 88 in FIG. 3. A time scale of ten nanoseconds is indicated in FIG. 4.
The actual numerical data provided in this reference indicate that a pumping pulse of forty nanoseconds duration and 260 millijoules energy is shifted and compressed to a first Stokes frequency pulse, shown in FIG. 4C, of six nanoseconds duration and 162 millijoules energy. Therefore, this reference teaches the use of a resonant cavity which is resonant at the pumping frequency, and contains a Raman medium in the form of pressurized gaseous hydrogen, in order to provide a frequency shifted, compressed pulse at the first Stokes frequency of the Raman medium.
In contrast, the first reference discussed a resonant cavity which is resonant at the first Stokes frequency for hydrogen, and the compressed pulse was obtained at the second Stokes frequency.
It is an object of the present invention to provide an improved optical frequency shifter and pulse compressor.